1. Field of the Invention
The invention relates to the fabrication of an electrical connector, and more particularly to a method of making a metal-on-elastomer connector containing vertically oriented thin wire filaments in an elastomeric mat.
2. Description of Related Art
Packaging components with high density pad configurations are often surface mounted on underlying interconnect structures such as substrates, printed circuit boards, and printed wiring boards. At times, electrical connection must be made between aligned opposing electrical contact areas. This is frequently the case with pad grid arrays (or land grid arrays) which contain flush contact areas. Opposing contact areas, however, may be difficult to solder, and may exhibit height variations from plating thicknesses, substrate warp, and non-planarities. Various connection schemes including high bump soldering have proven unreliable or expensive.
Elastomeric connectors have been developed for compliant high density interconnection which accommodates height variations between aligned opposing electrical contacts on two generally parallel surfaces. There are two basic types of metal-elastomer connectors: the layered elastomeric element and the elastomeric metal-on-elastomer. The layered elastomeric element comprises alternating layers of conductive and non-conductive silicone rubber, for instance 200 layers per inch.
Metal-on elastomer ("MOE") connectors, to which the present invention is directed, are now described. As seen in FIG. 1, the connectors contain vertically oriented (anisotropic) conductive filaments in a non-conductive elastomer. Metal filaments are normally preferred, but carbon fibers or conductive rubber rods may also be used. The filaments are separated and electrically isolated from one another, for instance 2 mil filaments on a 4 mil pitch, and may be distributed in linear, triangular, or square patterns. Thus, the connectors are electrically conductive in only one (Z-axis) direction and non-conductive in two (X- and Y-axis) directions. The elastomeric mat must maintain its spring force by virtue of it,s elasticity. Silicone rubber is the most widely used elastomeric material.
As seen in FIG. 2, a MOE connector is sandwiched between surfaces containing opposing electrical contacts. The opposing electrical contacts must be aligned with one another. However, since the area of the opposing contacts is much greater than the area of the wire filaments, the filaments need not be registered or aligned with the contacts. This highly significant feature is referred to as "redundant contact connection."
As shown in cross-section in FIG. 3, the components are mechanically secured together, the connector is compressed (e.g. 10%-40%), and the wire filaments provide electrical interconnection between opposing contacts. Only those filaments that touch the contacts provide paths for electrical conduction. A limited range of contact force is required to assure low contact resistance and vertical accommodation. By way of example, a clamping mechanism may apply 10 psi to compress the connector. Too small a force, such as 5 psi, may result in poor interconnection in areas of non-planarity; whereas too great a force, for instance 100 psi, may crush the connector.
In addition to vertical compliance, connection of aligned opposing contacts by MOE connectors has the advantages of simple mounting, removal and replacement, a wide range of geometries, lack of thermal stress from soldering, lack of chemical damage from fluxes or cleaning solvents, small pressures (10-20 psi), low inductance and low impedance. Furthermore, MOE connectors have been found to transmit high frequencies (2 GHz) without distortion, and to have low contact resistance (typically 10-100 milliohms).
Methods have formerly been developed in order to manufacture MOE connectors. Yonekura, "Oriented Wire Through Connectors For High Density Contacts," Nepcon West 1990, pp. 57-71 describes pre-bent wires oriented and embedded in a silicone elastomer. Zifcak et al, "Pinless Grid Array Connector," 6th Annual International Electronics Packaging Conference, Nov. 17-19, 1986, San Diego, Calif., pp. 453-464 uses a mechanically-frothed urethane foam with high retained stress in compression (i.e. low stress relaxation). The foam is machined to provide conductor openings and alignment holes. In particular, the conductor openings are produced by drilling two 0.020 inch diameter holes side-by-side at a 30 degree angle. The article also mentions conductor openings may be made by cutting, punching, or molding in place. Shaped rectangular conductors are then inserted in the conductor openings. Buchoff, "Elastomeric Connectors For Land Grid Array Packages," Connection Technology, April 1989, pp. 15-18 describes metal traces of gold on nickel on copper formed on the silicone rubber core surface. The article further describes using round wires which remain below the rubber surface during deflection, breaking contact. In "Matrix MOE Elastomeric Connectors," Technical Data Sheet, Elastomeric Technologies, Inc., the MOE's consist of gold conductive paths laminated to electrically insulating silicone. An additional technique known in the art is the use of magnetic levitation to orient ferromagnetic wires prior to curing an elastomeric material.
Therefore the related art does not teach how to manufacture metal-on-elastomer connectors in a relatively simple, low cost manner. The importance of MOE connectors in high density electronics packaging suggests a need for such a method.